The nature of ASASSN-24fw's occultation: modelling the event as dimming by optically thick rings around a sub-stellar companion

TL;DR

The study investigates ASASSN-24fw, a main-sequence F-type star that underwent an unusually long, flat-bottomed dimming event. By combining multi-wavelength photometry, spectroscopy, and a two-pronged lightcurve modelling approach (a Transit-Profile model and an Occulting-disc model), the authors argue that the dimming is caused by a sub-stellar companion with a massive circumplanetary ring system, rather than intrinsic stellar variability or circum-stellar material alone. The best-fitting geometry suggests a ring disc extending to ~0.17 au around a companion of at least ~3 Jupiter masses, consistent with a brown-dwarf or super-Jupiter. The work highlights the presence of gas and large bodies in the occulting structure, interprets the infrared excess as evidence for a complex circumstellar environment, and outlines future observations (JWST, polarimetry, high-resolution spectroscopy) necessary to constrain the system’s nature and evolutionary status.

Abstract

ASASSN-24fw is a main-sequence F-type star that experienced a rapid and long-lasting dimming event beginning in late 2024 and continuing until mid 2025. Its pre-dimming spectral energy distribution shows a persistent infrared excess with a fractional luminosity of approximately 0.5 percent. We model this excess using a two-component blackbody fit and find dust components with temperatures of about 1070 K and 390 K. Archival light curves indicate that ASASSN-24fw was photometrically stable prior to the event, suggesting that the dimming is caused by an external occulting body rather than intrinsic stellar variability. The event lasted about 275 days and exhibits a distinctive flat-bottomed profile of nearly 200 days, unlike most long-duration occultation events reported in the last decade. We analyze the light curve and spectra obtained during dimming to study the properties of both the star and the occulting material. A parametric light-curve model reveals multiple ingress phases, consistent with variations in the density and structure of the obscuring material. A second transit model favors an occulting body consistent with a gas giant or brown dwarf with a minimum mass of about 3.4 Jupiter masses and surrounded by an extended circumplanetary disk or rings of radius roughly 0.17 au. Near-infrared spectra taken during dimming show enhanced infrared excess and spectral features consistent with a late-type companion, approximately M8. We also detect variable H-alpha emission, suggesting evolving gas and dust in the occulting structure. Imaging from LCOGT identifies a nearby object within 3 arcsec, likely a bound companion at a projected separation of about 3000 au. Systems like ASASSN-24fw appear rare, and continued follow-up will help constrain the nature of the occulting body and the circumstellar environment.

Figures (4)

• Figure 1: LCOGT g-band post-dimming image of ASASSN-24fw in the center marked by an orange colored circle. The colorbar indicates the pixel value. We can also see a fainter companion inside the same circle which is at 3" separation in Gaia DR3. At a distance of $\simeq$ 1kpc, this companion is separated by $\simeq$ 3000 au.
• Figure 2: (Top-panel) Pre-dimming SED of ASASSN-24fw formed using various optical to infrared surveys photometry. The infrared excess can be seen towards the longer mid-infrared wavelengths which can be fitted by a two component blackbody model. (Middle-panel) Keck HiRES and Palomar TripleSPEC spectra that we obtained during-dimming. For reference, we have also plotted the M8 brown dwarf template in purple color, the continuum of which matches very well to the initial part of the observed NIR spectrum. The excess flux in the latter part of the NIR spectrum hints at elevated levels of infrared excess perhaps due to circum-planetary rings to the brown dwarf. (Bottom-panel) The residual plot where the three different Kurucz model spectra and the template brown dwarf spectrum are subtracted from the observed optical and NIR spectra.
• Figure 3: (a) Low resolution spectrum taken on 29th MONTH 2025 shows that the blue part of ASASSN-24fw is heavily extincted. The location of Hydrogen Balmer series, Ca H and K as well as Na Db lines are shown. (b) High-resolution and high SNR spectrum on 25th January 2025. Here Ca II H and K lines appear to be deeper than the Hydrogen Balmer series. The H$\alpha$ emission line is missing from both the spectra, confirming that ASASSN-24fw is not a YSO. Further, we see a weak CH G-band molecular line which is infact prominent in G-type and cooler stars. This indicates that ASASSN-24fw is a F-type star.
• Figure 4: (Top-panel) The Transit-Profile model fitted to the lightcurve of ASASSN-24fw. This helps us to statistically estimate the maximum depth of the dimming, duration of the dimming and quantify the shape of the inverted hat shape of the lightcurve. Here we notice the different slopes in the lightcurve which indicate that the dimming started with a $\simeq$ 150 days gradual decrease in the brightness, followed two stages of ingress. The wiggles or the "micro-trends" in the model fitted to the ASAS-SN lightcurve are because of the Gaussian Process regression trying to find a mean model through the distributed data. They do not appear in the fit to the ATLAS lightcurve due to difference in the distribution of the noise and data points but the "macro-trends" are still highlighted. (Bottom-panel) The occulting disc model generated using a central body with massive and optically thick circum-planetary rings, traversing the diameter of the star with a constant velocity and producing symmetrical lightcurve is shown. Although it misses the small features that the former model could show, from this model, we get an estimate on the geometry of the occulting system.